Electrostatic charge controller

ABSTRACT

Method and apparatus for controlling the electrostatic charge density on a surface by generating an ion current adjacent the surface, partially screening the surface from the generated ion current, and controlling the magnitude and sign of the ion current passing to the surface by means of an applied bias voltage. A charge sensor/feed back system may be employed to control the bias voltage.

United States Patent [1 1 Blythe et al.

[4 1 Jan. 28, 1975 ELECTROSTATIC CHARGE CONTROLLER [75] inventors: Anthony Reginald Blythe, Welwyn;

Wilson Reddish, London Colney, both of England [73] Assignee: Imperial Chemical Industries Limited, London, England [22] Filed: Jan. 4, 1974 [21] Appl. No: 430,626

Walkup 317/262 A De Gcest 317/2 R [57] ABSTRACT Method and apparatus for controlling the electrostatic [30] Foreign Application Priority Data Feb. 2. 1973 Great Britain 5236/73 Charge density on 11 Surface y generating an ion rent adjacent the surface, partially screening the sur- [52] [1.5. CI 317/2 R face from the generated ion curremand cmtmmng 511 Int. Cl. 110513/00 the magnitude and Sign of the ion Current p g w 58 Field of Search 317/2 R, 262 A the Surface y means of an pp bias volwge- A charge sensor/feed back system may be employed to 5 References Cited control the bias voltage.

1T T T S TE UN ED S A E PA NTS 3 Claims, 2 Drawing Figures 2,868,989 1/1959 Haacke 317/262 A l r I f ELECTROSTATIC CHARGE CONTROLLER This invention relates to electrostatically charged surfaces, and'to a method and apparatus for controlling, and, in particular, for reducing or eliminating, the electrostatic charge density on such a surface.

It is well known that frictional engagement of two surfaces creates electrostatic charges on those surfaces by a phenomenon, sometimes referred to as contact electrification, involving the transfer of electrons from one surface to the other. The resultant electrostatic charge is quickly dissipated on conductive surfaces such as metals, but may be retained for long periods where the surfaces are of insulating or semiconductive materials such as thermoplastic polymers.

The presence of an electrostatic charge on a surface is undesirable in that dust and other contaminants may be attracted to the charged surface. Furthermore, the processing and transporting of material in the form of webs, such as paper or thin polymeric films, is frequently hampered by the inherent tendency of the webs to adhere tightly to one another, and to other flat surfaces with which they come into contact, This phenomenon, commonly referred to as blocking, is at least partially accounted for by the electrostatic charges acquired by the web surface in the course of passing around the various drums and rollers employed in a web-forming andwinding process, and adversely affects the speed at which the winding operations can be conducted, hinders the production of uniformly slit web, and may induce creasing of the web. Additionally, the presence of electrostatic charges may initiate sparking with the attendant hazardsof fire and explosion, particularly in the presence of volatile solvents which may be employed in the production of coated webs.

The problems created by the presence of static charge on the surface of an article can, to some extent, be alleviated by treating the article with a conventional antistatic agent either in a coating composition applied to the surface, or by incorporation of the antistatic agent into the article itself. In some cases, however, the presence of an antistatic agent is undesirable for example, in polymeric films intended for use in electrical capacitors where the presence of an antistatic agent might be detrimental to the electrical characteristics of the film. it has therefore been proposed to neutralise static electricity by physical means involving the production of a stream of ionised air which is directed onto the electrostatically charged surface. Suitable equipment comprises an electrostatic eliminator consisting of a bar maintained at a high alternating potential and located within a semi-cylindrical case maintained at earth potential whereby ionised air, produced along the length of the bar, is directed against an electrostatically charged surface positioned at the open side of the earthed case.

The performance of eliminators of the kind referred to is somewhat variable, and, in particular, their efficiency tends to be impaired as they become contaminated with dust and other contaminants. In practice, therefore, it is difficult by the use of a conventional eliminator consistently to reduce the electrostatic charge density of a charged surface to less than about 1 picocoulornb per square centimetre of surface (pC/cm While this level of treatment is generally acceptable, there are various applications for which a much lower and consistently uniform level of electrostatic charge density is essential. For example, an extremely low level of charge density is essential for polymeric films on the surface of which electrostatic images are subsequently to be imposed for example, in electrostatic copying processes.

To achieve an acceptably low level of charge density it is necessary substantially to neutralize the final traces of electrostatic charge on a surface by depositing on that surface a small controlled charge of opposite sign to the charge already present on the surface.

We have now devised a way of imposing small. controlled amounts of charge onto an electrostatically charged surface.

Accordingly, the present invention provides a method for controlling the electrostatic charge density on a surface of an electrostatically charged article comprising establishing a corona discharge to generate an ion current adjacent said electrostatically charged surface, partially screening the charged surface from said ion current, and providing a bias voltage to direct a proportion of the generated ion current onto the charged surface.

The invention also provides an apparatus for controlling the electrostatic charge density on an electrostatically charged surface of an article comprising at least one corona discharge electrode to generate an ion current adjacent the charged surface, an ion currentattracting screen to be disposed between the electrode and charged surface, said screen being apertured to permit the passage therethrough of a proportion of the ion current, a backing member against which said article may be positioned or conveyed adjacent the screen, the backing member being spaced apart from the screen on the side thereof remote from the electrode, and means for supplying a bias voltage to control the magnitude and sign of the ion current passing to the electrostatically charged surface.

The techniques of the present invention are particularly suitable for the treatment of long lengths of films or webs, and for convenience of description the invention is hereinafter discussed in relation to the treatment of materials in film form, but it is to be understood that the techniques are also applicable to the treatment of other articles such as gramophone records, and thermoplastic containers such as bottles.

Generation of the ion current is conveniently effected by means of a longitudinal electrode extending transversely of the charged film surface. The electrode is suitably disposed at a distance of less than mm, preferably less than 50 mm, from the charged surface of the film to be treated, a separation of about 30 mm being particularly suitable, and is maintained at a suitably high potential, e.g., from about 3 to 10 kV AC, relative to the film surface to ensure that an electrostatic charge of the desired magnitude is deposited on the film. For certain applications the electrode may be maintained at a DC potential, but this suffers from the disadvantage that only a single sign charge can be deposited on the film. If desired an alternating potential of square wave form may be employed to facilitate efficient generation of the corona ion current. Where films moving at high linear speeds are to be treated, we prefer to provide a high frequency (e.g., up to about 10 MH supply to the electrode to reduce banding, i.e., the deposition of discrete bands of charge along the length of the film as a result of movement of the film between successive pulses of the ion current. Where a non-continuous surface is to be treated, e.g., a series of gramophone records, the supply should be pulsed so that a discrete quantity of ion current is generated to coincide with the positioning of each surface (record) against the backing member.

The electrode may extend continuously across the entire width of the film, or may be subdivided into a plurality of successive sections across the width of the film, each section being supplied from a separate source of potential to facilitate treatment of film having a non-uniform distribution of charge across the surface thereof. Suitably, the electrode is in the form of a series of spikes or needles arranged in one or more rows and directed towards the film. Alternatively, the electrode may comprise one or a series of knife-edges, e.g., razor blades, but conveniently is in the form of a wire, suitably a stainless steel or nickel-chrome wire of diameter from about 0.07 to 0.25 mm. As the thickness of the wire increases so also does the voltage necessary to establish a corona. At high voltages there is a danger of sparking to the surrounding case, and the working range is narrow. We therefore prefer to employ an electrode wire of diameter about 0.18 mm. If desired, a clean electrode wire may be continuously reeled across the film to avoid undue contamination of the electrode, as described in the complete specification of British Pat. No. 1,129,136. The electrode may also be heated if desired, to minimize contamination by condensation of vapours on the electrode, as described in the complete specification of British Pat. No. 1,248,807.

Preferably, the electrode is located within a partially enclosed earthed conductive casing for example a metal jacket, extending along the length of the electrode, the casing not extending between the electrode and film surface. The casing serves to stabilize the generated ion current, and to protect the operator from shock, and isconveniently of semicylindrical form, the diametrical face of the semicylinder being incomplete or removed in its entirety to allow the ion current generatedat the electrode to pass towards the screen, and thence to the film surface. Preferably, the casing is maintained at earth potential to improve the stability of the ion current.

The screen may be of any material which will attract the ion current from the electrode, and may for example, be fabricated from synthetic polymeric materials. However, we prefer that the screen should be of metal for example, aluminium or stainless steel, and should be maintained at earth potential. Conveniently, the screen may be integral with the casing. The use of an earthed metal screen is essential if the electrode is not provided with a conductive casing of the kind hereinbefore described.

Suitably, the screen is in the form of a fiat plate located substantially parallel to the charged film surface, at a convenient distance from both the electrode and the film surface. The distance of the screen from the electrode and film surface may be adjusted to give a balance in performance for positive or negative applied bias voltages, and, in practice, we have observed that the screen is conveniently located at a distance of some mm from both electrode and film surface.

The screen aperture is conveniently in the form of a longitudinal slit extending substantially parallel to the electrode, although any other suitable pattern, e.g. a

series of holes drilled in the screen plate, may be employed. if desired, the screen may be formed from two plates so arranged in the same plane that adjacent edges of the two plates define an appropriately dimensioned aperture, the aperture being preferably parallel to the electrode to provide uniform treatment of the film surface. The width of the aperture may be varied over a wide range to alter the characteristics of the system, but we prefer to employ an aperture width of not more than 10 mm, and preferably less than 5 mm. Thus, by employing an aperture in the form of a slit parallel to a wire electrode (0.1 13 mm diameter, 4kV AC), the slit having a width 1 mm, it has proved possible to achieve controlled ion currents in a range of from 0.04 to 0.4 nanoamperes (nA) per centimetre length of electrode with an applied bias of from 10 to volts.

The backing member against which the article or film may be positioned or conveyed during treatment is conveniently in the form of a stationary plate, curved or fiat, but, particularly for the treatment of long lengths of film or web like materials, we prefer that the backing member should comprise a moveable surface over and in contact with which the film or web may be transported. The moveable surface is suitably an endless belt, although a drum or roller is preferred. The surface of the latter is preferably highly polished, e.g., polished chrome steel, to prevent the film being damaged when it contacts the roller, but a matt-surface, or an insulating surface, such as a plastics-coated surface, may be employed.

As hereinbefore described, the backing member is conveniently disposed so that the charged film surface is located less than 100 mm, preferably less than 50 mm, and particularly at about 30 mm, from the electrode.

The moveable surface may operate to convey the film at any desired speed. Conveniently, the film is transported at a linear speed of from 1 to 6 metres per second.

Although the bias voltage may be applied in any convenient way for example, directly to the screen, we prefer to supply the bias voltage to the backing member.

Voltage is conveniently supplied to the backing member from a variable DC source, such as a potentiometer connected to a battery, or a DC voltage amplifier, the magnitude and sign of the applied voltage being determined by the magnitude and sign of the ion current desired to be attracted to the charged film surface. Although the applied bias voltage may vary over a wide range, we prefer that voltages of less than 100 volts should be employed. Desirably, the bias voltage should be kept as low as possible to enable a completely solid-state control system to be employed.

The system of the present invention is particularly suitable for the deposition of small amounts of charge onto a film which already has a relatively low surface charge density as a result of pretreatment of the film with a conventional charge eliminator. However, in order to neutralize the small remaining surface charge on a long length of pretreated film, we prefer, when winding the film into a roll, to employ our charging device in conjunction with a feedback control system, whereby the electrostatic charge density on the unwound film and/or the wound roll is measured, by conventional sensors, and the output from the sensors is used to control the bias voltage supplied to the charging device.

Polymeric films suitable for treatment by the techniques of the present invention include films formed from polycarbonates, polysulphones, polyamides such as polyhexamethylene adipamide or polycaprolactam, polyesters such as polyethylene terephthalate, polyethylene-l,2-diphenoxyethane-4, 4-dicarboxylate, and polyethylene-2,6-naphthalate, vinyl polymers and copolymers, polyolefins, and particularly a high molecular weight stereo-regular predominantly crystalline polymer of propylene, either in the form of a homopolymer or copolymerised with minor quantities (e.g., up to l5 percent by weight) of other unsaturated monomers such as ethylene.

Films are suitably formed from these materials in any conventional maner, as, for example, by rolling, extruding, pressing, solvent casting or melt casting techniques, and are preferably oriented by stretching the film to impart strength thereto. A particularly useful film is produced by melt extruding polypropylene in the form of a tube from an annular die, inflating the tube by the so called bubble" process to introduce transverse orientation, and simultaneously elongating the tube longitudinally to orient the film in a lengthwise direction. The oriented film may then be stabilised against thermal shrinkage by a heat-setting operation which involves heating the film, while held under tension, to a temperature above the glass transition temperature of the film-forming polymer but below the melting point thereof.

The techniques of the present invention are also applicable to the treatment of a metallised film, the film being metallised on one side only, for example metallised polypropylene film, and the electrostatic charge being deposited on the unmetallised surface of the film.

In the case of a metallised film, the bias voltage may, if desired, be applied directly to the metallised surface of the film.

Any of the aforementioned films may be metallised, for use in the process of this invention, by conventional metallising techniques, suitably by vacuum deposition in which a metal, such as aluminium, zinc, silver, gold or nickel, is evaporated onto a film located in a chamber maintained under conditions of high vacuum to deposit a layer of metal onto one surface of the film. Suitably, the film surface is subjected to chemical or physical treatments, for example, oxidation with mineral acids or exposure to a corona discharge, prior to metallising, thereby to improve the adhesion of the metallic layer to the film substrate.

The thickness of the article to be treated in accordance with the present invention is limited, inter alia, by the separation maintained between the screen and backing member. In the case of polymeric films we have found that treatment of films having a thickness of from 4 to about 150 microns is conveniently effected, although for use in packaging and capacitor dielectric applications we prefer to treat films of thickness from 6 to microns.

The electrostatic charge treatment of the film ac cording to the present invention may be effected at any convenient stage during the processing or winding of the film. For example, the charge may be deposited on a wide film which is being fed to a slitting device where the film is slit into several narrower strips each of a width suitable for winding into an electrical capacitor,

or other application, or a film may be electrostatically neutralised immediately prior to the deposition thereon of an electrostatic image. Packaging films may be subjected to the treatment prior to winding into rolls for storage and transportation.

Although a single charging device is normally adequate to provide the desired degree of electrostatic charge density on a film, a plurality of charging devices may be employed, if desired, to effect sequential treatment of the film. Sequential treatments may be effected on the same or opposite surfaces of the film, as desired. Conveniently, sequential treatment is effected so that at least one charging device deposits a positive charge, and at least one charging device deposits a negative charge onto the film surface.

If desired, an air blower, or the like, may be employed to direct the ion current onto the film surface, or to remove dust from the film surface after neutralization of the electrostatic charge thereon.

The invention is illustrated by reference to the accompanying drawings in which:

FIG. 1 is a perspective view of a film charging device according to the invention, and

FIG. 2 is a schematic view of a film charging device employed in conjunction with a charge sensing and feedback control system.

In the arrangement of FIG. 1, a polypropylene film 1 some 550 mm wide is shown passing in the direction of the arrows around and in contact with a backing member in the form of a chromium plated roller 2 of diameter 150 mm rotating at a peripheral speed of about 1.5 metres/second.

As the film passes in contact with the roller surface it is subjected to an ion current generated by a nickelchromium wire corona discharge electrode 3, 0.18 mm in diameter, extending transversely across the entire width of the film parallel to the roller axis at a distance of about 30 mm above the roller surface, and maintained at an electrical potential of 4kV AC supplied at a frequency of 50 Hertz from a mains transformer 4. The electrode is located within a semicylindrical aluminium casing 5 maintained at earth potential by an earthing device 6.

A screen consisting of two aluminum plates 7, 8, each about 1 mm thick, is located parallel to the axis of the roller some l5 mm above the surface thereof, and in electrical contact with the casing 5 so as to define a slot aperture 9 parallel to the electrode wire and of width 1 mm.

The ion current generated at the electrode wire is drawn to the earthed screen and a proportion thereof passes through aperture 9 to deposit an electrostatic charge on the surface of film 1, the magnitude and sign of the deposited charge being controlled by a bias voltage applied to roller 2 from a variable potential source 10. By varying the applied bias voltage between 10 and volts DC an electrostatic charge density of from 0.2 to 2.0 pC/cm can be deposited onto the film surface.

Referring to FIG. 2, a polypropylene film 21 is shown passing around an idler roller 22, thence in contact with a chromium plated roller 23 and subsequently being wound into a roll 24 on a core member 25. A screened, wire electrode film charging device, depicted generally at 26, is located adjacent roller 23, the spacing of the electrode and shield in relation to the roller surface being similarto that of FIG. 1.

An electrode probe 27 senses the net charge density on the untreated film, and the output signal from an associated electrometer 28 is fed to a summing amplifier, depicted generally at 29. A second electrode probe 30 senses the integrated charge density accumulated on roll 24, and the output signal from associated electrometer 31 is differentiated in a differentiator, depicted generally at 32, and subsequently fed to summing amplifier 29, where the two output signals are combined to provide a bias voltage V of the form,

where A and B are circuit constants. The variable bias voltage V is then applied to roller 23 whereby the accumulated charge density in roll 24 can be maintained at an acceptably low level.

The control system may be modified so that either the first electrode probe 27, or the second electrode probe 30, is dispensed with, the signal from the remaining electrode probe being employed to effect appropriate control of the applied bias voltage, and thus to control the accumulated charge on the wound roll 24.

The invention is illustrated by reference to the following Examples.

EXAMPLE 1 Using a charging assembly of the kind shown in H6. 1 an initially uncharged film of polyethylene terephthalate some 12 microns thick and 213 mm (8.375 inches) wide was wound around and in contact with an electrically insulated chromium plated roller of 1 14 mm (4.5 inches) diameter rotating at a peripheral speed of 0.05 ms" (l0ft/min).

As the film passed around the roller it was subjected to an ion current generated at a nickel-chromium wire corona discharge electrode 0.076 mm (0.003 inches) in diameter extending transversely across the entire width of the film parallel to the roller axis at a distance of 26.7 mm (1.05 inches) above the roller surface and maintained at an electrical potential of 4 kilovolts AC. The electrode was located within an earthed semicylindrical casing and was partially screened from the film by two rectangular aluminium plates of thickness 0.8 mm (0.03 l 25 inches) so positioned at the mouth of the casing as to define a slot aperture of width 2 mm (0.08 inches) parallel to the electrode wire and some mm (0.39 inches) above the roller surface.

A variety of DC bias voltages were applied to the roller to control the amount of electrostatic charge deposited by the ion current onto the travelling film, and the corresponding charge density on the film surface was measured downstream of the roller by a conventional charge meter.

Results are recorded in the accompanying table.

Table-Continued Applied Bias Density of Voltage Deposited Charge (volts) (pC/cm) +307 285 +371 --360 0 0 "370 +248 3l0 +2l2 -259 +170 l l28 +95 64 +50 EXAMPLE 2 Using a charging assembly identical to that of Example l a polyethylene terephthalate film some 12 microns thick and 213 mm (8.375 inches) wide, and having an electrostatic charge density of pC/cm was wound around the backing roller rotating at a peripheral speed of 0.05 ms (IOft/min).

The DC bias voltage applied to the roller was varied until the charge density of the film, recorded downstream of .the roller, was zero, i.e., until the initial charge on the film had been neutralized. The required bias voltage was -275 volts.

EXAMPLE 3 The procedure of Example 2 was repeated using a polyethylene terephthalate film having an initial electrostatic charge density of +125 pC/cm and travelling over the backing roller at a linear speed of 0.1 ms (20ft/min). The required bias voltage to neutralize the film was -250 volts.

We claim:

1. A method for reducing the electrostatic charge density on an electrostatically charged surface of a thermoplastic polymeric film comprising establishing a corona discharge to generate an ion current adjacent said electrostatically charged surface, partially screening the charged surface from said ion current, sensing the electrostatic charge density on the charged surface, generating a signal proportional to the sensed charge density, providing a bias voltage of less than 100 volts at a surface of the film remote from that exposed to the corona discharge to direct a proportion of the generated ion current onto the charged surface and employing said generated signal to control the bias voltage at a value such that the electrostatic charge density on the charged surface is reduced to less than 1 picocoulomb per square centimetre of surface.

2. A method according to claim 1 wherein the film of a thermoplastic polymeric material is selected from the group consisting of polycarbonates, polysulphones, polyamides, polyesters and polyolefins.

3. A method according to claim 1 comprising winding said signal to control the bias voltage. 

1. A method for reducing the electrostatic charge density on an electrostatically charged surface of a thermoplastic polymeric film comprising establishing a corona discharge to generate an ion current adjacent said electrostatically charged surface, partially screening the charged surface from said ion current, sensing the electrostatic charge density on the charged surface, generating a signal proportional to the sensed charge density, providing a bias voltage of less than 100 volts at a surface of the film remote from that exposed to the corona discharge to direct a proportion of the generated ion current onto the charged surface and employing said generated signal to control the bias voltage at a value such that the electrostatic charge density on the charged surface is reduced to less than 1 picocoulomb per square centimetre of surface.
 2. A method according to claim 1 wherein the film of a thermoplastic polymeric material is selected from the group consisting of polycarbonates, polysulphones, polyamides, polyesters and polyolefins.
 3. A method according to claim 1 comprising winding the treated film into a reel, sensing the electrostatic charge density of the reeled film, generating a signal proportional to the sensed charge density, and employing said signal to control the bias voltage. 